The HSV-1 mechanisms of cell-to-cell spread and fusion are critically dependent on host PTP1B.

Jillian C Carmichael,John W Wills,Anthony Schmitt,Rebecca C Craven,Hiroki Yokota,Eain A Murphy

doi:10.1371/journal.ppat.1007054

Abstract

All herpesviruses have mechanisms for passing through cell junctions, which exclude neutralizing antibodies and offer a clear path to neighboring, uninfected cells. In the case of herpes simplex virus type 1 (HSV-1), direct cell-to-cell transmission takes place between epithelial cells and sensory neurons, where latency is established. The spreading mechanism is poorly understood, but mutations in four different HSV-1 genes can dysregulate it, causing neighboring cells to fuse to produce syncytia. Because the host proteins involved are largely unknown (other than the virus entry receptor), we were intrigued by an earlier discovery that cells infected with wild-type HSV-1 will form syncytia when treated with salubrinal. A biotinylated derivative of this drug was used to pull down cellular complexes, which were analyzed by mass spectrometry. One candidate was a protein tyrosine phosphatase (PTP1B), and although it ultimately proved not to be the target of salubrinal, it was found to be critical for the mechanism of cell-to-cell spread. In particular, a highly specific inhibitor of PTP1B (CAS 765317-72-4) blocked salubrinal-induced fusion, and by itself resulted in a dramatic reduction in the ability of HSV-1 to spread in the presence of neutralizing antibodies. The importance of this phosphatase was confirmed in the absence of drugs by using PTP1B-/- cells. Importantly, replication assays showed that virus titers were unaffected when PTP1B was inhibited or absent. Only cell-to-cell spread was altered. We also examined the effects of salubrinal and the PTP1B inhibitor on the four Syn mutants of HSV-1, and strikingly different responses were found. That is, both drugs individually enhanced fusion for some mutants and reduced fusion for others. PTP1B is the first host factor identified to be specifically required for cell-to-cell spread, and it may be a therapeutic target for preventing HSV-1 reactivation disease.

Highlights

There are two ways that viruses can spread to uninfected cells
It is estimated that 67% of the global population is infected with herpes simplex virus type 1 (HSV-1)
This virus resides in sensory neurons in a quiescent state but periodically reactivates, producing virus particles that travel down the axon to infect epithelial cells of the skin, where it can be transmitted to additional people

Summary

Introduction

Cell-free spread occurs when virions are released from an infected cell into their surrounding environment prior to entering a new cell. This, is how viruses spread to new hosts and often between cells within a host. Some viruses, including all the herpesviruses, have a “cell-tocell” spreading mechanism by which virions pass directly through cell junctions, enabling protection from neutralizing antibodies [1, 2]. Herpes simplex virus type 1 (HSV-1) utilizes cell-to-cell spread to move directly from mucosal epithelial cells, the initial site of infection, into nearby sensory neurons, where the virus establishes a latent infection. Replication-competent mutants of HSV-1 that are defective for cell-to-cell spread fail to infect neurons when tested in animal models, and cannot establish latency [5, 6]. The mechanism of cell-to-cell spread remains poorly understood for all herpesviruses

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